Computer guided cryosurgery

ABSTRACT

A system for assisting surgeons in performing cryosurgery of the prostate by calculating optimal positions for cryoprobes and providing display based templates for overlay over an ultrasound image display, and displaying actual cryoprobes ultrasound images together with template images so that the surgeon may compare suggested and actual placement of the cryoprobes, and adjust placement accordingly.

CROSS REFERENCE

This application is a continuation of U.S. application Ser. No.09/318,710, U.S. Pat. No. 6,139,544 filed May 26, 1999.

FIELD OF THE INVENTIONS

The inventions described herein relate to the field of cryosurgery andablative surgery.

BACKGROUND OF THE INVENTIONS

The system and methods described below enhance the accuracy andeffectiveness of cryosurgery of the prostate. Cryosurgery of theprostate is an effective treatment for prostate cancer and benignprostate hyperplasia, conditions which affect many men.

The use of cryosurgical probes for cryoablation of the prostate isdescribed in Onik, Ultrasound-Guided Crvosurgerv, Scientific American at62 (January 1996) and Onik, Cohen, et al., Transrectal Ultrasound-GuidedPercutaneous Radial Cryosurgical Ablation Of The Prostate, 72 Cancer1291 (1993). In this procedure, generally referred to as cryoablation ofthe prostate, several cryosurgical probes are inserted through the skinin the perineal area (between the scrotum and the anus) which providesthe easiest access to the prostate. The probes are pushed into theprostate gland through previously placed cannulas. Placement of theprobes within the prostate gland is visualized with an ultrasoundimaging probe placed in the rectum. The probes are quickly cooled totemperatures typically below −120° C. The prostate tissue is killed bythe freezing, and any tumor or cancer within the prostate is alsokilled. The body will absorb some of the dead tissue over a period ofseveral weeks. Other necrosed tissue may slough off through the urethra.The urethra, bladder neck sphincter and external sphincter are protectedfrom freezing by a warming catheter placed in the urethra andcontinuously flushed with warm saline to keep the urethra from freezing.

To maximize the effectiveness of the procedure, the entire prostateshould be ablated. At the same time, surrounding structures such as therectum and the neurovascular bundles should not be frozen. The amount ofthe prostate which is ablated by the cryosurgical procedure depends onthe number of cryoprobes used and their placement within the prostategland. Wong, et al., Cryosurgery as a Treatment for Prostate Carcinoma,79 Cancer 963 (March 1997), suggests a placement scheme for cryosurgicalprobes within the prostate. Probes were inserted through the perinealarea into the prostate while attempting to keep the probes within 1.8 cmof each other. The systems and methods presented below were developed toassist surgeons in placing the probes as suggested by Wong, or assuggested by others, with the assistance of ultrasound imaging, computergraphics, and computer assisted calculations of optimal probe placementwithin the prostate.

SUMMARY

The inventions described below are designed to assist surgeonsperforming cryoablation of the prostate gland in a male human patient.The system includes an ultrasound imaging probe and associated imageprocessing hardware and software and image display systems, a computersystem which generates a user interface which accepts input from asurgeon as to the size and shape of the prostate gland imaged by theultrasound imaging system, and calculates an optimal cryosurgical probeplacement for the particular imaged prostate gland, and displays atemplate on the display screen indicating the optimal placement. Thesystem also images the cryosurgical probes as they are inserted into theprostate gland, and presents the images of the cryosurgical probes onthe display for comparison with the template and the optimal placementpositions as calculated by the system. Using the system, the surgeonperforming the procedure can be assured that the intended probeplacement corresponds to optimal positions, and that the actual probeplacement is accomplished according to the optimal placement.

Also described are algorithms and a corresponding computer programdesigned to calculate the optimal position of the cryoprobes foreffective cryosurgical ablation of the prostate in a wide range ofpatients. The algorithms decide whether the prostate size fits withinparameters for successful calculations, whether five or six probes arerequired, the optimal placement for two cryoprobes in the anterior lobeof the prostate gland, the optimum placement for two cryoprobes in theouter portions of the posterior lobe of the prostate, and the optimumplacement for one or two cryoprobes in the center area of the posteriorlobe of the prostate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an overview of the transperineal cryosurgical ablation of theprostate.

FIG. 2 is an illustration of a sagittal or longitudinal cross section ofthe prostate and surrounding anatomy as displayed on the video displayprovided by the system.

FIG. 3 is an illustration of a horizontal or transverse cross section ofthe prostate and surrounding anatomy as displayed on the video displayprovided by the system.

FIG. 4 is an illustration of the system in use to outline the prostateimage shown in FIG. 2.

FIG. 5 is an illustration of the system in use to outline the prostateimage shown in FIG. 2, illustrating measurements calculated by thecomputer system.

FIG. 6 is an illustration of the system in use to outline the prostateimage shown in FIG. 3.

FIG. 7 is an illustration of the system in use to outline the prostateimage shown in FIG. 3, illustrating measurements calculated by thecomputer system.

FIG. 8 is an illustration of the system determination of the optimumplacement of cryoprobes within the anterior lobe of the prostate imageshown in FIG. 3, where the prostate is relatively large.

FIG. 9 is an illustration of the system determination of the optimumplacement of two cryoprobes within the anterior lobe of the prostateimage shown in FIG. 3, where the prostate is relatively large.

FIG. 10 is an illustration of the system determination of the optimumplacement of two cryoprobes within the anterior lobe of the prostateimage shown in FIG. 3 where the prostate is relatively small.

FIG. 11 is an illustration of the system determination of the optimumplacement of two cryoprobes within the posterior lobe of the prostateimage shown in FIG. 3.

FIG. 12 is an illustration of the system determination of the optimumplacement of a fifth cryoprobes within the prostate image shown in FIG.3, in the case that only five probes are required for effectivecryosurgical ablation of the prostate.

FIG. 13 is an illustration of the system determination of the optimumplacement of the fifth and sixth cryoprobes within the prostate imageshown in FIG. 3, in the case that a sixth probe is required foreffective cryosurgical ablation of the prostate.

FIGS. 14 through 21 illustrate a second method for the systemdetermination of the optimum placement of cryoprobes within theprostate.

FIG. 22 is an illustration of the system output indicating the optimumplacement of the fifth and sixth cryoprobes within the prostate imageshown in FIG. 3, in the case that a sixth probe is required foreffective cryosurgical ablation of the prostate.

FIG. 23 is an illustration of the system output indicating the optimumplacement of cryoprobes within the prostate image shown in FIG. 2.

FIG. 24 is an illustration of the system output indicating the optimumplacement of cryoprobes within the prostate image shown in FIG. 2.

FIG. 25 is an illustration of the system output indicating the actualplacement of cryoprobes in relation to the displayed optimum placementof cryoprobes within the prostate image shown in FIG. 2.

FIG. 26 is an illustration of the system output indicating the actualplacement of cryoprobes in relation to the displayed optimum placementof cryoprobes within the prostate image shown in FIG. 2, displayingprobes and graphical markers obscured in the view of FIG. 25.

FIG. 27 is an illustration of the system output indicating the actualplacement of cryoprobes in relation to the displayed optimum placementof cryoprobes within the prostate image shown in FIG. 3.

FIG. 28 is a copy of the portion of the computer program which performsthe calculations used to determine the optimum placement of cryoprobes.

DETAILED DESCRIPTION OF THE INVENTIONS

FIG. 1 shows one of the basic operations for which the cryoprobes aredesigned. Several probes 1, 2, 3, 4 and 5 are shown inserted in theprostate 6 (in some instances a sixth probe, not shown, is used). Allfive probes are inserted through the perineal region 7 between thescrotum and the anus. Probes 2 and 3 are shown inserted into theanterior lobe 6 a of the prostate, and Probes 1, 4 and 5 are showninserted into the posterior lobe 6 p, which is larger than the anteriorlobe. The probes are placed within the prostate according to procedureswell known in the art, and a suitable procedure is described instep-by-step detail in Onik, et al., Percutaneous Prostate Cryoablation,(1995) at pages 108-112 and Onik, Ultrasound-Guided Cryosurgery,Scientific American at 62 (January 1996). Typically five or six probesare used in the procedure, though more or less may be used when requiredby unusual anatomy of a particular patient. The urethra 8 which passesthrough the prostate is one of the anatomic structures that usuallyshould not be frozen during this surgery. Accordingly, the urethra isprotected and kept warm with the urethral warming catheter 9. Thebladder neck sphincter 10 and the external sphincter 11 are alsostructures that should be protected from freezing, and these areprotected from freezing by the warming catheter. Neurovascular bundleson the right and left of the prostate should also be protected fromfreezing. Transrectal probe 13 is inserted into the rectum 14 in orderto visualize the placement of the probes and the growth of the iceballsformed by the cryoprobes. To assist in placement of the cryosurgicalprobes, a template 15 is used which supports the probes during insertionand while they are installed in the body. The patient is placedhorizontally on an operating table with legs positioned to provideaccess for the ultrasound probe to be inserted into the rectum andcryoprobes to be inserted through the perineal area into the prostate.

The transrectal ultrasound probe 13 is used to visualize the prostateand the cryosurgical probes. The ultrasound probe operates in the rangeof about 2-10 MHz, depending on the equipment used (the processdescribed herein may be used with any ultrasound probe and ultrasoundgenerator, which may be selected based on various technical, medical andbudgetary considerations). The image is displayed as a two dimensionalrepresentation of the boundaries of the prostate, as illustrated inFIGS. 2 and 3.

FIG. 2 shows a longitudinal (also referred to as sagittal or coronal)cross section of the prostate, generally denoted by the ultrasoundoutline 21. The top, bottom, anterior and posterior sides of the outlineare delineated by the letters T, B, A and P, respectively. The apex ofthe prostate 22 points generally to the perineal area shown in FIG. 1.Other anatomical structures visible in display include the urethra,whose outline 23 may not appear fully in the coronal cross section. Theurethral warming catheter image 24 appears within the urethra. Also, theultrasound image of the rectal wall 25 appears along the bottom of thedisplay area, and the image of the ultrasound probe 26 may appeardirectly below the rectal wall. The rectum and rectal wall arepreferably straightened by the pressure exerted by the insertion of theultrasound probe, as explained below, to set up the system for thecalculations and displays described below. Since the prostate isgenerally oblong in this direction, and longer in this direction than inthe horizontal or transverse cross section, we refer to this crosssection as the longitudinal cross section.

FIG. 3 shows a horizontal or transverse cross section of the prostategenerally denoted by the ultrasound outline 28. The anterior (towardsthe front of the body), posterior (toward the back), right and left ofthe prostate are marked as A, P, R and L, respectively. The urethraappears as the ultrasound outline 29. The warming catheter outline 24appears within the urethra image 29. Also visualized in the display isthe shadow of the probe itself, marked as item 30, and the ultrasoundoutline of the rectum 31. The image shown is refreshed at a regular rateby the ultrasound imaging system and the images may shift with movementof the ultrasound probe. The surgeon is instructed to translate theultrasound probe within the rectum to obtain several cross sectionalviews and to choose the largest viewable cross section of the prostatefor analysis and display.

Other aspects of the display shown in FIGS. 2 and 3 are provided as partof the user interface. The ultrasound display area 32 is used by thesystem to display the image of the prostate generated by the ultrasoundimaging system, and is used as described below to enter informationregarding the prostate outline and display suggested probe placements.The toolbar area 33 across the top of the screen is used by the systemto present drawing and viewing tools and other interface tools (typicalselection, pen and zoom tool icons 34, 35 and 36 are illustrated in thetoolbar). The data presentation area 37 along the right side of thescreen is used by the system to indicate program information and patientinformation, step by step instructions to the surgeon, and measurementsderived from operator inputs in the display area.

The system provides a function for the operator to freeze the image inorder to accept outlining and path-finding inputs followed bycalculating functions. The operator is prompted to orient the ultrasoundimage such that the rectal wall is substantially parallel to the bottomedge of the display area. The operator is instructed or prompted tosearch for the largest cross section of the prostate as seen through theultrasound probe, and to freeze the image on the screen. In response tothe operator's instruction to the system to freeze the images, a singleframe will be captured or grabbed by the system software and displayedas a frozen image. The frozen images are presented on the display toallow the operator to interact with the system to determine the size andshape of the prostate.

FIG. 4 shows the grabbed image of the coronal cross section of theprostate shown in FIG. 2. First viewing the live image of FIG. 2 toinspect a number of cross sections along different planes, the surgeoninstructs the system to freeze the image when an appropriate crosssection is displayed. Generally, the largest visualized cross section inwhich the urethra appears should be used. Visualization of the urethraindicates that the image plan is at or near the center of the prostate.When instructed do so, the computer system grabs a frame from the liveimage, and presents a frozen image as illustrated in FIG. 4.

To determine if the cross section is acceptable and that the ultrasoundprobe is placed in the preferred position, the operator marks at leasttwo points 39 and 40 along the rectal wall image 25 and instructs thecomputer system to analyze these two points to make sure that they aresufficiently horizontal (parallel with the ultrasound probe) andstraight to support later functions of the system. If the rectal wallimage is not horizontal within acceptable limits (currently, not morethan 4 mm difference in height on the display area), and near the bottomof the display area within acceptable limits (currently set at 3 cm),the system will reject the frozen image as a basis for later functionsand prompt the operator to correct the image orientation, probeplacement or patient position. If the rectal wall image is substantiallyhorizontal on the display, indicating that the rectal wall is parallelwith the ultrasound probe, the system will indicate that the image isacceptable and prompt the operator to continue the procedure and outlinethe prostate image on the display.

Using outline functions of the supporting computer system, such as thepen tool selected from the toolbar, the operator outlines the image ofthe prostate by moving the pen tool around the outline and anchoring thepen tool at selected points around the perimeter until the prostate isfully outlined. (The anchor points may be created by the operator withstandard input devices such as a mouse or touch screen and otherstandard software tools such as an anchor point adding tool, pen tool orpolygon tool.) The operator then creates an outline of the coronal crosssection of the prostate by drawing a polygon having several anchorpoints 41, 42, 43, 44, 45, 46, and 47 around the perimeter of theprostate ultrasound image 21 (any number of anchor points may be used).The operator indicates to the system that the outline is accuraterelative to the displayed image, and the computer system accepts theimage as a representation of the coronal cross section of the prostate.

Referring to FIG. 5, the computer system searches the display data forthe outline and defines the two parameters H2 and L2. The parameter H2is the “height” of the prostate that will be considered by the system.The parameter L2 is the length of the prostate that will be consideredby the program. (It should be appreciated that any variable name may beassigned to these parameters; the H2 and L2 designations correspond tovariables used in the computer program which the inventors have devisedto implement the system.) At this point, the computer system analyzesthe variables to assist the operator in deciding how to accomplish thecryosurgery. If L2 is greater than 35 mm, the system will notify theoperator that a pullback freeze is required to completely ablate theprostate. The doctor will then be apprised that a single freezingoperation will be insufficient, and that the cryosurgery must beaccomplished in two steps, with a first freeze being accomplished withthe probe tips near the top of the prostate and the second freeze beaccomplished afterward, with the cryoprobes pulled back about 10 mmtoward the apex of the prostate. If H2 is less than a predetermineddistance from either end of the prostate, the system will prompt theoperator to verify that the outline accurately reflects the size andshape of the prostate, whereupon the process may continue or berestarted.

Next, the operator performs the outlining task in relation to thehorizontal cross section. Thus, FIG. 6 shows the display of FIG. 3modified by the operator with the addition of a polygon comprisingseveral anchor points 48, 49, 50, 51, 52, 53 and 54 around the perimeterof the prostate ultrasound image 28 (any number of points may beentered). The operator indicates to the system that the outline isaccurate relative to the displayed image, and the computer systemaccepts the image as a representation of the coronal cross section ofthe prostate. The operator is also prompted to mark the center of theurethral prostate, and enters a mark 55, which the computer systemaccepts as the location of the prostatic urethra in the horizontal crosssection.

As shown in FIG. 7, the computer system searches the display data forthe outline and defines the two parameters H1 and L1. The parameter H1is the “height” of the prostate that will be considered by the system,as determined in the horizontal cross section image. The parameter L1 isthe width of the prostate that will be considered by the program. Afterthe parameter H1 is determined, the computer system will compare it withthe parameter H2 (the height as determined in the coronal cross sectionimage as shown in FIG. 5). These two parameter are expected to be thesame if the operator has actually selected the largest identifiablecross section in both planes. If H1 is different from H2 by more that apredetermined amount, this indicates that the cross sections do notclosely correspond to the largest actual cross section, and the computersystem will prompt the operator to restart the procedure at the imageselection stage (presently, 3 mm difference is the maximum alloweddifference, but this may vary with experience with the present system,or with the cryoprobes in use). Upon determining L1, the computer willcompare it with a predetermined width, and if L1 is greater than thepredetermined width (currently set at 3.5 cm) the system will notify theoperator that six cryoprobes are required for the operation, and proceedas described below to calculate a position for the sixth cryoprobes.

The system will not calculate positions for extremely large or smallglands, thus if the prostate width in the anterior lobe is greater than54 mm, the system will display a message in the instruction area thatthe gland is too large for calculated positioning of the probes. If theprostate width in the anterior lobe is smaller than 23 mm, the systemwill display a message in the instructions area that the gland is toosmall for calculated positioning of the probes.

The supporting computer system is programmed with software which permitsthe drawing and outlining functions described above, and calculationfunctions for calculating the size of the prostate based on the input,determining the optimal number of probes required, and determining theoptimal placement for the probes. When the outline of the prostate hasbeen entered into the computer system, and the computer system haschecked that the various parameters are within predetermined ranges, thecomputer system operates to calculate the optimum number of cryoprobesand the optimum placement of the cryoprobes, and communicate thisinformation to the operator.

The computer system requires input or calculation of certain parametersin order to determine probe placement. The parameters required are:

H1: Maximum front to back thickness in the horizontal cross section;

L1: Maximum right to left thickness in the horizontal cross section;

H2: Maximum front to back thickness in the sagittal cross section;

L2: Maximum right to left thickness in the sagittal cross section;

prtUrethra: The location of the center of urethra within the horizontalcross section;

iY0: the vertical position of the anterior extreme of the prostate glandin the horizontal cross section;

iY01 and iY5: The vertical position of the rectal wall in the horizontalcross section, as approximated by the posterior extreme of the prostategland;

The measured parameters are used to calculate the optimum position forthe placement of the cryosurgical probes. The optimum placement ofprobes is determined empirically, based upon histological evidence oflethal ablation zones, and the calculations may vary as histologicaldata accumulates. The system first calculates the position of probes inthe horizontal plane, and from this derives the position of the probesin the coronal cross section.

Using data from the outlining step, the system first calculates thedesired position of probes 2 and 3, which are placed in the anteriorlobe of the prostate. Referring to FIG. 8 (horizontal cross section), ifL1 exceeds 3.6 cm, the computer system searches the horizontal outlineof the prostate (using a line by line measurement of the display datagenerated by the computer system) from the top to identify the firsthorizontal length which equals 3.6 cm. When this line is found, asexemplified by line 57, the line is divided into four sections of equallength, and the five points P1, P2, P3, P4, and P5. The system nexttests whether the distance from point P2 to the midline highest point 58(pctrpt) is less than 9 mm. If this distance is less than 9 mm, thesystem uses this vertical position for placement of the probes 2 and 3.Graphical markers corresponding to Probes 2 and 3 are placed on thepoints P2 and P4, which correspond to the middle points on the two linesegments from the outer edge of the prostate to the center of theprostate at the point where the width of the prostate first measures 3.6cm. If this distance is initially more than 9 mm, the system moves theline 57 upward until the distance is 9 mm, and adjusts P1 through P5 tomatch the outer edge of the prostate, the centerline of the prostate,and the points between the center and the outer edges, and uses this newvertical position for placement of probes 2 and 3. This is illustratedin FIG. 9, where line 57 a has been raised to a higher level in thedisplay so that distances 59 and 60 are 9 mm. Graphical markerscorresponding to Probes 2 and 3 are placed on the points P2 and P4,which now correspond to the middle points on the two line segments fromthe outer edge of the prostate to the center of the prostate at thepoint where the width of the prostate first measures 3.6 cm, after thisline segment has been raised to the point where the middle points are 9mm from the centerline front of the prostate. Thus, in the case wherethe prostate width is greater than 3.5 cm, the first two graphicalmarker probes are placed by the system on the display in optimalposition.

In the case that L1 is less than 3.5 cm, placement of the first twoprobes is determined as illustrated in FIG. 10. The computer systemsearches the horizontal outline of the prostate (using a line by linemeasurement of the display data generated by the computer system) byiteratively drawing horizontal lines and identifying the five points P1through P5, from the top to identify the first horizontal line in whichthe distance point P2 to the top centerline is 9 mm, or until thedistance between P2 and P4 is 9 mm, whichever occurs first. When thisline is found, as exemplified by line 61, the points P2 and P4 areidentified as the position for Probes 2 and 3, respectively.

With the positions for Probes 2 and 3 determined, the system nextdetermines the optimal position for probes 1 and 4, which are placed inthe posterior lobe of the prostate. The procedure is explained inreference to FIG. 11. The system finds all points at which the distancealong the horizontal from the outer edge of the prostate (distance b) isone half of the distance along the vertical from the outer edge of theprostate (distance a). All the points along lines 63 and 64 (bothrepresenting line b=1/2a on opposite sides of the prostate) illustratedin FIG. 11 fit this criterion (note that the line is not necessarilystraight, since it is defined in reference to the irregular outline ofthe prostate). The system next draws the arc 65 at a distance of 18 mmfrom point Probe 2, and a similar arc 66 from point Probe 3. The pointwhere the arc 65 and the line 63 intersect is chosen as the position ofprobe 1 and the point where the arc 66 and the line 64 intersect ischosen as the position of probe 4. Currently, these probes arepositioned in this manner regardless of prostate size, except asinherited from the positioning of probes 2 and 3.

Placement of probe 5 depends on the size of the prostate. If the systemhas decided that 5 probes are sufficient to accomplish effectiveablation, the fifth probe is placed on the line H1 between the rectalwall and the urethra. The system finds the point along H1 which is 2/3of the way up from the rectal wall, and assigns this position to Probe5, as illustrated in FIG. 12. If the system has decided that 6 probesare necessary (i.e., L1 is greater than 3.6 cm), the system determinesthe position of probes 5 and 6. As illustrated in FIG. 13, the systemdetermines the position of these probes relative to previously placedprobes. The system determines the position of probe 5 so that it fallswithin 1.8 cm of probe 1, and the distance from the horizontalcenterline L1 to the probe is not more than 1 cm, and the distancebetween from the vertical center line H1 is not more than 9 mm. Arc 67is drawn around Probe 1 with radius of 18 mm, and a line 68 is drawn 1cm below L1. Probe 5 is placed on the higher of (1) the intersection ofline 69 (drawn 9 mm away from H1) with the arc 67, or (2) theintersection of the line 69 and line 68. A similar construction isperformed on the other side of the prostate, and probe 6 is placedwithin the intersection of line 70 and the arc 71 or the line 68,whichever is higher. Thus Probes 5 and 6 are located on the screenrelative to the ultrasound image of the prostate.

FIGS. 14 through 21 illustrate a second method for the systemdetermination of the optimum placement of cryoprobes within theprostate. Referring to FIG. 14, the prostate outline is used as beforeto define H1 and L1. The system finds the top of the prostate outline,and sets the variable iY0 at the top of prostate (represented by thehorizontal line iY0), and sets the variable pctrpt as the x and y valuesat the top of the prostate. Minimum and maximum potential verticallevels are defined for analysis as locations for probes 2 and 3. Thesystem defines the variable ip2ymin at 8 mm below the line iY0 (labeledas horizontal line ip2ymin), and defines the variable ip2ymax at highestof (1) 16 mm below iY0 (labeled as horizontal line iY0+16 mm) and (2) 4mm above the center of the urethra (labeled as horizontal linepctUrethra-4 mm), and defines the variable ip2ylimit (the lowest allowedheight for probes 2 and 3) as the horizontal line 7/16 down H1 (labelediP2ylimit). The system searches line on the screen starting at ip2yminand continuing through to the highest of ip2ymax or ip2ylimit, whichever is higher. (If iP2ymin is lower than iP2ymax, the system setsip2ymin as the y value for probes 2 and 3.)

Referring now to FIG. 15, the system searches the region defined in FIG.14 for the desired vertical placement of probes 2 and 3. The systemdetermines the length of ip2ymin, and the length of line ip2ymax acrossthe prostate. If upper line (iP2ymin) length (dpt1topt2min) is greaterthan 54 mm, the outline cannot be automatically analyzed because it istoo big, and the system communicates this to the operator. If the lowerline (iP2ymax) length (dpt1topt2max) is smaller than 23 mm, the outlinecannot be automatically analyzed because it is too small, and the systemcommunicates this to the operator. In all other instances, the systemcontinues on to create and define variables as follows:

For L1 between 26 and 36 mm, create variable pt1topt2, and store theminimum value of 26 or dpt1topt2max (length of ip2ymax line) aspt1topt2;

For L1 between 36 and 54 mm, store the minimum of 36 or dpt1topt2max(length of line iP2ymax) as pt1topt2;

For L1 between 54 nd 58 mm, store the minimum of 44 or dpt1topt2max aspt1topt2;

For L1 greater than 58 mm, store the minimum of 54 or dpt1topt2max aspt1topt2.

Now referring to FIG. 16, the system searches for the proper height forprobes 2 and 3. From the top line (iP2ymin), the system scans each pixelline from iP2ymin to iP2ymax, searching for outside points X1 and X5 onthe outline intersecting the horizontal line. When a line is found wherethe length of this line equals the defined dPt1toPt2 (calculated inreference to FIG. 15, above), that line is used as the verticalplacement of probes 2 and 3. The corresponding line in FIG. 16 islabeled 72. Finally, the system calculates the x values for probes 2 and3, setting them along the line at a spacing of 1/4 and 3/4 from theleft, respectively. Thus, the system determines the position of twoprobes in the anterior lobe of the prostate based upon the width of theprostate approximately at the anterior-most location of (1) 16 mmposterior to the anterior extremity of the prostate, (2) 4 mm anteriorto the center of the prostatic urethra, or (3) 7/16 of the totalthickness of the prostate from the anterior extremity of the prostate.

Now referring to FIG. 17, the system analyzes the outline to determinethe proper location of probes 1 and 4 in the posterior lobe of theprostate. The system defines the vertical location iY01 (horizontal lineiY01 in FIG. 17) as the y value or line at 5/8 down the prostate (5/8down the length of H1). The system determines the Y distance betweeniY01 line and probes 2 and 3 (about 10 mm, as illustrated). If thisdistance is less than 16 mm, the system uses iY01 line as verticalheight for placement of probes 1 and 4. Here, iY01 is only 10 mm away,so this is the height for probes 1 and 4. If the distance is greaterthan 16 mm, the system raises line iY01 until it is only 16 mm down fromprobes 2 and 3. To determine the horizontal placement of probes 1 and 4,the system places point iXL 1/6 across iY01 line, and if greater than 18mm from probe 2, slide right (or left) until the point is 18 mm fromprobe 2 (i.e., within circle 73, drawn at a radius of 18 mm from probe2). Similarly, the system places point iXR 5/6 of the distance acrossthe line iY01, and if greater than 18 mm from probe 3, the point ismoved left (or right) until the point is 18 mm from probe 3 (i.e.,within circle 74, drawn at a radius of 18 mm from probe 3). Asillustrated, both points fall within 18 mm of the probe above, so pointsiXL and iXR are used as the points for suggested placement of probes 1and 4 respectively. FIG. 18 illustrates the same procedure in relationto a larger prostate which requires sliding point iXR to the left afterinitial placement at 5/6 across line iY01, so that it comes within 18 mmof probe 3 (and meets circle 74). In all other respects, FIG. 18 isdescribed in the same manner as FIG. 17. Thus, the system determines theposition of two probes in the posterior lobe of the prostate based uponthe width of the prostate at the anterior-most location of (1)approximately 5/8 of the total thickness of the prostate from theanterior extremity of the prostate or (2) 16 mm posterior to the twoprobes positioned in the anterior lobe of the prostate.

Next, the system determines the placement of Probes 5 and 6 within theposterior lobe of the prostate. Referring to FIG. 19, the system setsline iY01 as y height of probes 1 and 4, and sets the horizontal lineiY56 at 1/4 up from bottom of screen (prostate/rectum/screen bottom arein reality so close together that they may be treated as the same ylevel), and sets the line (labeled as the horizontal line iY01+16) 16 mmdown from probes 1 and 4. Then, the system resets line iY56 at thehighest (the minimum y value) of the two lines, here the 1/4 up line ischosen. The system places points X5 and X6 at left and right extremitiesof line iY56, which is the intersection of the prostate outline withiY56. To place the probes 5 and 6 at the appropriate horizontallocation, the system moves points X5 and X6 inward toward H1 until theyare 10 mm apart. FIG. 20 illustrates the placement of probes 5 and 6 ina larger prostate, to illustrate the system choice of the highest linefor placement of the probes. Here, line iY56 is below line iY01+16, soline iY56 is reset to the height of line iY01+16. Once again, the pointsX5 and X6 are moved inward along the higher line until they are only 10mm apart, and probes 5 and 6 are located at these points, as illustratedin the figure. Thus the system determines the position of the two probes5 and 6 in the center portion of the posterior lobe of the prostatebased upon the width of the prostate at a the anterior-most location of(1) approximately 1/4 of the distance on the display from the rectum tothe top of the display; (2) 16 mm posterior to the two probes 1 and 4already positioned in the posterior lobe of the prostate.

In a prostate where L1 is less that 35 mm across, only probe 5 is used,and it is located at or near the horizontal center of the prostateoutline. As illustrated in FIG. 21, the system sets iY5 at bottom ofoutline. The system also defines the horizontal line 1/3 the way down H1from the urethra to rectum (the horizontal line labeled aspctUrethra+1/3), and defines the horizontal line 18 mm above iY5 (thehorizontal line labeled iY5−18). The system then selects the lowest ofthese two lines as the vertical position for probe 5. Probe 5 is placedhorizontally on the horizontal midpoint between probes 1 and 4 (expectedto be on or near H1). Thus, as an alternate to placement of both probes5 and 6, the system places probe 5, based upon the width of theprostate, in the center portion of the prostate at the posterior-mostlocation of (1) 1/3 the distance posterior from the urethra to therectum or (2) 18 mm anterior to the posterior extremity of the prostate

After calculation of the optimal placement of the cryoprobes accordingto either of the methods above, the system graphically displays thedesired locations to assist the operator in placing the actual probes inthe prostate of the patient. The optimal location of the probes isindicated in the horizontal cross section by a graphic representationoverlaid over the live ultrasound images of FIGS. 3 and 2 and/or thestill images of FIGS. 6 and 4. The desired representation is illustratedin FIGS. 22 and 25. FIG. 22 is an illustration of the system outputindicating the optimum placement of cryoprobes within the prostatehorizontal cross sectional image shown in FIG. 2. The suggested probeplacement is indicated by graphical markers 75, 76, 77, 78, 79 and 80for Probes 1, 2, 3 4, 5 and 6 respectively. The markers are placed inthe display by the computer system, overlaying the ultrasound image ofthe prostate horizontal cross section.

The system also provides an illustration of the placement of the probesin the sagittal cross section. FIGS. 23 and 24 show the displaypresented by the computer system of graphical markers for each probe.The vertical position of the probes is dependent on the positionscalculated in reference to calculated positions in the horizontal crosssection. FIGS. 23 and 24 show the system output indicating the optimumplacement of cryoprobes within the prostate longitudinal cross sectionalimage shown in FIG. 3. The suggested probe placement, as viewed on thesagittal plane, is indicated by graphical markers, 81, 82, 83, 84, 85and 86. The markers are placed in the display by the computer system,overlaying the ultrasound image of the prostate sagittal cross section.Since probes 2 and 3, and probes 1 and 4, and probes 5 and 6 are likelyto be symmetrically located in reference to the line H1, the probes inthese pairs will overlap in this view. Thus, probes 3, 4 and 6 aredisplayed as shown in FIG. 24. The computer system permits the operatorto switch between views of FIG. 23 and FIG. 24. FIG. 23 is used by theoperator to assist in placement of Probes 1, 2 and 5, while FIG. 24 isused to assist in placement of Probes 3, 4 and 6.

With the optimal probe placements calculated and graphical markersplaced on the display, the operator may insert cryoprobes into theprostate. FIGS. 25, 26 and 27 illustrate the feedback provided to theoperator indicating the actual position of the cryoprobes in relation tothe suggested placement shown in FIGS. 22, 23 and 24. Referring to FIG.27, the graphical markers 75, 76, 77, 78, and 80 are shown in thehorizontal cross section, displayed as generated by the computer system.In addition, the ultrasound image of the probes is displayed in thedisplay area, since the probes enter the ultrasound imaging field andare imaged by the ultrasound imaging system. As the surgeon inserts eachprobe into the prostate, its placement as indicated by the ultrasoundsystem may be compared to the suggested probe placement, and the surgeonmay manipulate the probes so that the ultrasound images of the actualprobes align with the graphical markers. In FIG. 27, the exemplaryplacement of probes is illustrated. Probe 1 has been placed in goodcorrespondence with the template provided by the computer, and theultrasound image is aligned with the graphical marker 75. Probe 2 hasbeen placed in a position different than its associated graphical marker76, and the surgeon may decide on that basis to reinsert the probe tomore closely align it with the marker. Probe 3 has been placed close tothe marker 77, and the surgeon may decide to reposition the probe or toleave it in place. Likewise, Probes 4 and 5 appear in the display on ornear their associated markers 78 and 80, providing feedback to thesurgeon ensuring proper placement of the probes.

Referring to FIG. 25, the graphical markers 81, 82 and 85 are shown,displayed as generated by the computer system. In addition, theultrasound image of the probes is displayed in the display area, sincethe probes enter the ultrasound imaging field and are imaged by theultrasound imaging system. Again, the surgeon may view the ultrasoundimage of the actual probes, and place the probes as closely as possiblepositions corresponding to the markers. FIG. 26 shows a displaycontaining the graphical markers 83 and 84 corresponding to probes 3 and4 which are located on the right side of the prostate relative to theanterior/posterior centerline. These displays help the operator inplacing the probes as desired in parallel relationship with theultrasound probe and the rectal wall. The operator may switch repeatedlybetween the displays of FIGS. 25, 26 and 27 while inserting thecryoprobes, selectively displaying the image of the horizontal crosssection and the image of the coronal cross section, to monitor theprogress of the probes and ensure placement of the probes isaccomplished in the positions suggested by the computer system. Whencryoprobes placement is satisfactory, the surgeon will start the flow ofcooling gas to freeze the prostate. The freezing operation can beconfirmed in the ultrasound image by watching the iceballs (the mass offrozen tissue) around each cryoprobes form. The extent of the iceballsand the extent of the prostate that is frozen is monitored to ensurethat substantially all of the prostate is frozen. The freezing processmay be repeated to ensure ablation of the prostate.

The section of the computer program which performs these calculations isprovided as FIG. 28, which is programmed in the C++ programminglanguage. The program implements the second method described above inrelation to FIGS. 14 through 22. The section of the code which finds theproper vertical location for probes 2 and 3 in the anterior lobe of theprostate starts at item number 90. The section of code which determinesif the gland is too large or small is indicated by item number 91. Thesection of the code which finds the proper horizontal and verticallocation for probes 2 and 3 in the anterior lobe of the prostate islabeled as item 92. The section of the program which calculates theposition of probes 1 and 4 is indicated by item number 93. This segmentof code also incorporates a test for adequate symmetry of the prostate,in that if probe 1 or probe 4 cannot be properly place, the systemcommunicates this to the operator (leading to re-imaging or cancellationof the computer assisted surgery). The section of the program whichcalculates the position of probes 5 and 6 (or only probe 5) is indicatedby item number 94.

Thus, we have described a system for assisting surgeons in performingcryosurgery of the prostate by calculating optimal positions forcryoprobes and providing display based templates for overlay over anultrasound image display, and displaying actual cryoprobes ultrasoundimages together with template images so that the surgeon may comparesuggested and actual placement of the probes, and adjust placementaccordingly. The method and system is described above in relation to ourCRYOcare™ cryosurgical system, which is provided with up to eightindependently controlled 3 mm argon powered cryoprobes. The system coolsthe probes to cryosurgically effective temperatures (typically below−120° C.) through Joule-Thomson cooling within the probe tips. Thesystem may be implemented with other cooling systems such as liquidnitrogen cryoprobes and mixed gas cryoprobes. The placement of probes iscalculated based on this system, and the calculations may be adjustedfor different systems and numbers of probes. Additionally, while thesystem has been described with ultrasound as the imaging mechanism andthe rectum as the point of view, the system may be implemented withother imaging systems such as fluoroscopy or new imaging systems. Thesystem may be adapted to other forms of ablation and treatment of theprostate or other organs, with adjustments in the calculations beingmade to account for the ablative range of the devices used and thegeometry of the organ. Thus, while the preferred embodiments of thedevices and methods have been described in reference to the environmentin which they were developed, they are merely illustrative of theprinciples of the inventions. Other embodiments and configurations maybe devised without departing from the spirit of the inventions and thescope of the appended claims.

We claim:
 1. A method for assisting a surgeon in placing cryoprobes in the prostate of a human patient, wherein the cryoprobes are inserted through the skin of the perineal area of the patient and into the prostate, including the anterior lobe of the prostate and the posterior lobe of the prostate, said method comprising the steps of: providing a first cryoprobe, a second cryoprobe, a third cryoprobe, a fourth cryoprobe, a fifth cryoprobe and a sixth cryoprobe, and a cooling system for cooling the cryoprobes to cryosurgically effective temperatures; providing an ultrasound imaging system including a transrectal ultrasound probe and ultrasound display; said ultrasound imaging system being capable of creating a first image of the prostate on a first plane and a second image of the prostate in a second plane; providing a computer system capable of accepting input from an operator identifying the outline of the prostate in reference to the first image and the second image of the prostate displayed by the ultrasound imaging system; said computer system being programmed with software capable of performing the following steps; computing an optimal position for placement of a first cryoprobe and second cryoprobe in the anterior lobe of the prostate gland; computing an optimal position for placement of a third cryoprobe and fourth cryoprobe in the posterior lobe of the cryoprobe; computing the need for the sixth cryoprobe, and upon determining the need for the sixth cryoprobe, computing an optimal position for placement of the fifth cryoprobe and the sixth cryoprobe in the posterior lobe of the prostate; alternately, upon determining that a sixth cryoprobe is unnecessary, computing an optimal position for placement of the fifth cryoprobe in the posterior lobe of the prostate; displaying a first set of graphical markers on the display of the first image at positions corresponding to the computed optimal positions for each of the first cryoprobe, the second cryoprobe, the third cryoprobe, the fourth cryoprobe, the fifth cryoprobe and the sixth cryoprobe; displaying a second set of graphical markers on the display of the second image at positions corresponding to the computed optimal positions for each of the first cryoprobe, the second cryoprobe, the third cryoprobe, the fourth cryoprobe, the fifth cryoprobe and the sixth cryoprobe; permitting the operator to selectively display the first image and the second image.
 2. A method for assisting a surgeon in placing cryoprobes in the prostate of a human patient, wherein the cryoprobes are inserted through the skin of the perineal area of the patient and into the prostate, including the anterior lobe and the posterior lobe of the prostate, said system comprising a display for displaying an ultrasound image of the prostate, a template of suggested cryoprobes placements in the prostate, and images of cryoprobes placed within the prostate, and a computer system programmed to compute the position of the cryoprobes with a method comprising the steps of; determining the width of the prostate based on the image, and determining the number of cryoprobes required based on the width of the prostate; determining the position of two cryoprobes in the anterior lobe of the prostate based upon the width of the prostate approximately at the anterior-most location of (1) 16 mm posterior to the anterior extremity of the prostate, (2) 4 mm anterior to the center of the prostatic urethra, or (3) {fraction (7/16)} of the total thickness of the prostate from the anterior extremity of the prostate; determining the position of two cryoprobes in the posterior lobe of the prostate based upon the width of the prostate at the anterior-most location of (1) approximately ⅝ of the total thickness of the prostate from the anterior extremity of the prostate or (2) 16 mm posterior to the two cryoprobes positioned in the anterior lobe of the prostate; determining the position of two cryoprobes in the center portion of the posterior lobe of the prostate based upon the width of the prostate at a the anterior-most location of (1) approximately ¼ of the distance on the display from the rectum to the top of the display or (2) 16 mm posterior to the two cryoprobes positioned in the posterior lobe of the prostate; alternately, based upon the width of the prostate, placing a single cryoprobes in the center portion of the prostate at the posterior-most location of (1) ⅓ the distance posterior from the urethra to the rectum or (2) 18 mm anterior to the posterior extremity of the prostate; wherein the positions determined by the computer system are displayed as graphical markers displayed with the ultrasound image of the prostate, and ultrasound images of cryoprobes are displayed with the ultrasound image of the prostate, thereby allowing the surgeon to compare actual cryoprobes placement with optimal cryoprobes placement as determined by the computer system.
 3. A system for assisting a surgeon in placing a surgical instrument in an organ of a human patient through a placement point, said system comprising: imaging means being capable of creating two-dimensional images of the organ on a plurality of planes; and creating images of the surgical instrument in place within the organ; and a template superimposed over the images of the organ, said template having a marker for pinpointing the desired placement point of the surgical instrument in the organ; wherein the two-dimensional images of the organ, the surgical instrument displayed, and the marker are displayed as a two-dimensional representation to allow the surgeon to compare actual instrument placement with desired instrument placement.
 4. The system of claim 3, wherein the imaging means is an ultrasound imaging system including an ultrasound transducer and ultrasound display.
 5. The system of claim 3 wherein the template and marker are created by a computer system coupled to the imaging means.
 6. The system of claim 3 wherein the template and marker are created by the surgeon.
 7. A computer system having a display for assisting a surgeon place cryoprobes in the prostate of a human patient, wherein the cryoprobes are inserted though the skin of the perineal area of the patient and into the prostate, including the anterior lobe of the prostate and the posterior lobe of the prostate, the display adapted to display images of the prostate, said computer system being programmed with software capable of performing the following steps; computing an optimal position for placement of a first cryoprobes and second cryoprobes in the anterior lobe of the prostate; computing an optimal position for placement of a third cryoprobes and fourth cryoprobes in the posterior lobe of the cryoprobes; computing the need for the sixth cryoprobes, and upon determining the need for the sixth cryoprobes, computing an optimal position for placement of the fifth cryoprobes and the sixth cryoprobes in the posterior lobe of the prostate; alternately, upon determining that a sixth cryoprobes is unnecessary, computing an optimal position for placement of the fifth cryoprobes in the posterior lobe of the prostate; displaying a first set of graphical markers on the display at positions corresponding to the computed optimal positions for each of the first cryoprobes, the second cryoprobes, the third cryoprobes, the fourth cryoprobes, the fifth cryoprobes and the sixth cryoprobes.
 8. A system for assisting a surgeon in placing a plurality of cryoprobes in the prostate of a human patient, wherein the cryoprobes are inserted through the skin of the perineal area of the patient and into the prostate, including the anterior lobe of the prostate and the posterior lobe of the prostate, said system comprising: an ultrasound imaging system including a transrectal ultrasound probe and ultrasound display; said ultrasound imaging system being capable of creating images of the prostate on a plurality of planes and said ultrasound imaging system being capable of creating images of the cryoprobes; a computer system capable of accepting input from an operator identifying the outline of the prostate in reference to the image of the prostate displayed by the ultrasound imaging system; said computer system being programmed with software capable of performing the following steps; computing an optimal number of cryoprobes; computing optimal positions for placement cryoprobes of said cryoprobes in the prostate; displaying graphical markers on the display at positions corresponding to the computed optimal positions for each of the cryoprobes; permitting the operator to selectively display from the plurality of planes.
 9. A system for assisting a surgeon in placing cryoprobes in the prostate of a human patient, wherein the cryoprobes are inserted through the skin of the perineal area of the patient and into the prostate, including the anterior lobe of the prostate and the posterior lobe of the prostate, said system comprising: an ultrasound imaging system being capable of creating images of the prostate on a plurality of planes and creating images of the cryoprobes; a template having markers corresponding to the desired placement of each cryoprobes in the prostate; and a display adapted to display the images of the prostate and the images of the cryoprobes; and display the template and the markers superimposed over the images of the prostate and the images of the cryoprobes.
 10. The system of claim 9 wherein the template and markers are created by a computer system coupled to the ultrasound imaging system.
 11. The system of claim 9 wherein the template and markers are created by the surgeon. 